Cheaters method:
If you are using a firm to hard mousse, you can use the same pans you used for the cake..........
Wash and dry the cake pan......OR.....You can use the same size cheesecake/springform pan.
Spray it lightly with non-stick spray, lightly wipe with paper towel.
Cut out waxed paper or baking parchment paper to fit the pan, bottom and sides....the light coating of non-stick spray is so the paper will stick to the pan so you won't have issues when putting the mousse in.
Place pan in freezer while you make the mousse.
Depending on how many layers of filling you need, will depend on how many pans you will need, or how many times you will have to do this if you only have one pan.
Once your mousse is ready, take the pan out of the freezer and pour in 1/2 an inch to 1 inch mousse, and flatten it out with a spatula.
Cover pan with plastic wrap and place in coldest part of fridge...usually top shelf all the way in back.
Once the mousse has firmed up, take it out of the pan, turn it onto a piece of plastic wrap, peel the paper off............ you will need to place it between your cake layers as soon as you can, because working with it will make it go limp quickly. Pick it up from under the plastic wrap and place one layer of cake on top of the mousse and then flip the whole thing over onto your serving tray or cake plate.
Place the other cake layer on top and frost it.
This method can also be used for ice cream cakes, using ice cream instead of mousse and placing it in the freezer over night instead of the fridge. Also hard custard or any firm filling.
**Note that if making a stacked cake, use thick dowel rods for stability and so the cake wont slide from the weight. It is better to make the different cake sizes on tier platforms rather than make a stacked cake.